1. Field of the Invention
The present invention relates generally to electric power steering apparatuses for use in automotive vehicles which provide an electric steering assist of an electric motor to the vehicle steering system to reduce a steering effort that is to be manually applied by a vehicle operator or driver. More particularly, the present invention relates to an improved electric power steering apparatus which, in case of an anomaly or abnormal condition in a microprocessor constituting a main power steering control, turns off or deactivates the motor to prevent the motor from producing an abnormal steering assist.
2. Description of the Related Art
Various types of electric power steering apparatuses have been proposed and known today. One of the known types of electric power steering apparatuses is designed in such a manner that, once manual steering torque is applied beyond a predetermined dead zone (namely, a zone where no steering assist is produced by an electric motor, i.e., no current flows through the electric motor, in spite of a driver's steering operation to the left or right from the center or neutral position), the apparatus inhibits the operation or driving of the electric motor in a direction opposite to the direction of the driver-applied manual steering torque and prevents an abnormal target motor current from being generated due to an abnormal condition in a microprocessor constituting a main power steering control for controlling the overall operation of the motor.
More specifically, in the electric power steering apparatus of the above-mentioned type, when the microprocessor has got into an abnormal condition to generate a maximum target motor current in a single direction (e.g., rightward) irrespective of presence or absence of driver-applied manual steering torque, the motor is caused to rotate rightward. If the driver is holding the steering wheel at that time, leftward steering torque would be detected and the operation of the motor is inhibited once the detected leftward steering torque exceeds a threshold value of the dead zone. Then, as the electric motor is deactivated, the detected steering torque decreases to enter the dead zone, so that the motor is again turned on or activated and thus the detected steering torque in again increases. Such occurrences are repeated, which would cause an undesired "hunting" of steering torque in the neighborhood of the dead zone, thus creating a possibility of unstable steering characteristics.
Japanese Patent Laid-open Publication No. HEI-8-108856 discloses an electric power steering apparatus which is designed to provide a solution to the undesired hunting. Specifically, the disclosed electric power steering apparatus detects a motor current in an opposite direction to a direction of steering torque or abnormal motor current and triggers a timer upon detection of the motor current of the opposite direction to the steering torque or abnormal motor current. Then, upon lapse of a time period preset in the timer, a latch circuit is activated and a motor drive inhibition circuit is activated in response to an output signal from the latch circuit so that a motor driver circuit is deactivated to stop the motor current of the opposite direction to the steering torque or abnormal motor current. The motor drive inhibition circuit remains activated as long as the latch circuit is activated, to thereby stop the motor current of the opposite direction to the steering torque or abnormal motor current. The activated condition of the motor drive inhibition circuit continues until the latch circuit is deactivated by a non-driven state detection circuit detecting that the target motor current has reached a zero (0) level or value.
However, because of the arrangement that the motor drive inhibition circuit is activated upon lapse of the timer-set time period after detection of the motor current of the opposite direction to the steering torque or abnormal motor current, the disclosed electric power steering apparatus encounters the problem that such a motor current of the opposite direction to the steering torque or abnormal motor current would undesirably continue flowing during the timer-set time period.
Further, because the latch circuit is deactivated when the zero target motor current is detected by the non-driven state detection circuit, the disclosed electric power steering apparatus has another problem that, even when the normal operation of the microprocessor is restored, the target motor current would not decrease to the zero value as long as the steering wheel is being operated to produce steering torque, so that the latch circuit, and hence the motor drive inhibition circuit, would remain activated to thereby keep disabling the motor driver circuit. Further, when steering torque is being generated during the activation of the latch circuit, a target motor current is generated with no driving current actually flowing through the motor, and thus, in a situation where an offset between the target motor current and the actual detected motor current is subjected to PID (Proportional, Integral and Differential) control operations, the duty ratio would reach almost 100% due to the I (Integral) control operation. Consequently, the motor would be driven at the 100% duty ratio to create a possibility of an overcurrent flowing through the motor the moment the latch circuit is deactivated.